California's underwater terrain: sandstones, folds, and a very active fault

 

Another connection-following journey, entertaining, informative - and time-consuming. For his 12th day of Christmas post, back on January 5th, Chris Rowan of Highly Allochthonous wrote of "Twelve folds a-plunging," describing, and giving some dramatic examples of, the natural calligraphy of large-scale folding of the earth's crust; here's one that he provided from Australia. 

 

I enjoyed this post immensely, having at one stage of my life been a structural geologist and one who continues to take great delight from the artistic and humbling manifestations of deformation and mountain-building. Then, recently, I was catching up on some of the remarkable imagery that's available through Google Earth's "underwater terrain" facility; having written about the Monterey Submarine Canyon, I immediately zoomed in to some of the incredible high-resolution data from the floor of the canyon that has been provided by the Monterey Bay Aquarium Research Institute. I have to admit that Google's insistence on including a sort of "sunlight reflecting off moving virtual waves" effect is irritating and can obscure some of the detail, but nevertheless it's impressive. Having scrutinised the canyon, I then took off northwards, looking for other areas of high-resolution images along the California coast, and was astonished when I came to what were clearly "folds a-plunging" in the sea-floor off San Mateo County - the overall image is at the top of this post. My reaction was "Wow - what is going on here?" and that, of course, precipitated a bit of a research journey.

This is Half Moon Bay, Seal Cove, and Moss Beach. The geology, like that of most of California, is complicated - and dynamic. But, of course and as usual, the USGS came through and provided, online, downloadable, and in great detail, the answers to my questions (see the reference at the end of the post). Situated west of the San Andreas fault, this region is sliced by a number of related faults, all of which are active; the map below summarises the main faults (the numbers are their inferred rate of movement in millimetres per year).

The San Gregorio fault cuts right across Half Moon Bay, across the promontory where the airport is located, and then heads out to sea again to the northwest. Here, thanks to the USGS, is the relevant part of a detailed geological map of the area.

 

The pinkish area to the northeast is underlain by granite, the yellow and beige areas along the coast are successions of relatively young and modern marine terraces, sands and shales of beaches, intertidal, and shallow marine deposits recording changing sea levels and the ups and downs of the land over the last few hundred thousand years. But these sediments have not had a peaceful post-depositional existence: the continuing movement of the faults of the San Andreas system, and here in particular the San Gregorio Fault (locally known as the Seal Cove fault), have not only shifted them sideways and vertically, but they have squeezed the rocks into large folds that can be defined where they occur onshore. On the geological map above, the oval-shaped pattern along the coast southwest of the fault is a plunging fold. Here's an extract from the USGS guidebook to the area:

Continuing activity along the Seal Cove Fault is ominously revealed by a fault scarp that offsets the young Half Moon Bay Terrace near the Half Moon Bay Airport. The elevation of the airport runway is about 40 feet, whereas the high point west of the fault scarp is about 175 feet. The Half Moon Bay marine terrace formed when coastal erosion cut a broad bench during a Late Quaternary marine transgression. The flat, wave-cut surface of the marine terrace became exposed as sea level fell and the shoreline retreated seaward about 85,000 years ago. The marine terrace has remained exposed as the coastline (and the entire Santa Cruz Mountains) has progressively continued to rise. The Half Moon Bay Terrace is actually folded into a northwest trending, gently-plunging syncline. Since the marine terrace became exposed, the vertical component of offset along the Sea Cove Fault has produced the 140 foot high scarp that is visible today. The fault places latest Quaternary deposits adjacent to Pliocene-age marine mudstones of the Purisima Formation. Note that the rate of horizontal slip on the fault is probably ten to twenty times as great compared
to the vertical offset during the same time period.

And, along the coast at Moss Beach, at low tide, this is what can be seen: 

 

The Moss Beach Syncline, a broad fold, is exposed at low tide in the James V. Fitzgerald Marine Preserve. Folded sedimentary layers of the plunging syncline consist of fossilerous sandstone and mudrocks of the Purisima Formation (Miocene to Pliocene age). The Seal Cove Fault scarp is associated with a submerged ridge located near where the waves break just offshore of the syncline. [USGS]

And that, in exquisite detail, is exactly what the Google Earth sea-floor imagery is showing: resistant sandstone ridges, intensely folded and faulted.

What is more, whereas the geological map inevitably had to just speculate on the continuation offshore of the fault (the dotted lines and the question marks), here it shows up with incredible clarity, a long linear feature on the sea bed, disrupting the pattern of folds. So here I am, sitting at my laptop in London, interpreting the offshore geology of California......

I've put together an image of the whole area, with what I would interpret as the trace of the fault between the arrows (but, of course, I would welcome alternative viewpoints!).

This is a dramatically active, but not very well-understood fault system. The residents of the mobile-home park clearly situated across the fault next to the airport runway have probably not read the guidebook quoted above or the paper by Gary Simpson and his colleagues published some time ago in the Bulletin of the Seismological Society of America, the abstract of which reads as follows:

The San Gregorio fault is the principal active fault west of the San Andreas fault in central coastal California, yet it remains the largest known fault in the region whose seismogenic potential is not known. In this study, we integrate traditional paleoseismic and archaeologic investigations to define the location, style, and timing of slip events on the northern San Gregorio fault at a site near Seal Cove in Moss Beach, California. The on-land portion of the San Gregorio fault at Seal Cove is a late-Holocene active dextral slip fault. Trench excavations revealed a broad zone of faulting, at least 22 m wide, consisting of five Holocene-active strands. These include a single mid-Holocene east-vergent reverse fault and four late-Holocene near-vertical strike-slip faults. The most recent event occurred after the deposition of a native Californian cooking hearth dated A.D. 1270 to A.D. 1400, but prior to the arrival of Spanish missionaries ca. 1775. The penultimate event at the site is less well constrained but appears to have occurred between A.D. 620 and A.D. 1400. The penultimate event was associated with horizontal displacement on the order of 3 m, based on reconstruction of a thrust wedge within the fault zone. The geometry of midden deposits shows a 5 (–2, +6) m deflection along the projection of faults associated with the most recent event (MRE). All or part of this deflection may be associated with the MRE. These displacements are consistent with M_w 7 to 7.25 earthquakes and show that the San Gregorio fault is an active seismogenic source that should be considered in seismic hazard assessments in the San Francisco Bay area.

Since I posted this, I have received a report from a local resident that there are plans under review for a 300,000 square feet complex to be constructed immediately next to the mobile home park - and the fault. See the website Montara Fog for more information - and note that the reported in the video would appear to be standing on the fault scarp.

[For an excellent description of the geology of the area that has provided much of the material for this post, see the USGS guidebook http://pubs.usgs.gov/of/2005/1127/; the complete geological map can be found at http://pubs.usgs.gov/of/1998/of98-137/, together with the explanation and other material. The reference for the paper on the San Gregorio fault is:   Gary D. Simpson et al., "The northern San Gregorio fault zone: Evidence for the timing of late Holocene earthquakes near Seal Cove, California"  Bulletin of the Seismological Society of America; October 1997; v. 87; no. 5; p. 1158-1170]

Comments

[sourceoutdoor]

While you are sitting in London, interpreting the offshore geology of California... I am sitting in Portland, Maine, reading your fascinating post. I am just starting my studies in Geology and so am always looking online for more information than is given to us in school. I like reading geologist's actual experiences and thoughts. Though I have not heard the sound of your knees, I'll have to go and read your book now. Thanks you! :)

[holy land tours]

Excellent work done!!!These pictures gives very clear view..California's underwater terrain so well explored.I am not so interested in geology but after seeing all this i am looking forward to have more information about it...Moss Beach picture showing low tide quite innovatively..i like this post..and wait for more such information about such places..

[temecula chiropractor]

The Half Moon Bay marine terrace formed when coastal erosion cut a broad bench during a Late Quaternary marine transgression. The flat, wave-cut surface of the marine terrace became exposed as sea level fell and the shoreline retreated seaward about 85,000 years ago.

[quail hill homes]

Situated west of the San Andreas fault, this region is sliced by a number of related faults, all of which are active